This invention relates to fiber optic cable and to apparatus and methods for installing fiber optic cables in a paved roadway or other similar surface or into solid ground.
U.S. Pat. No. 6,371,691 discloses a method for introducing a fiber optic cable in solid ground, such as a paved roadway, with the aid of a laying unit that includes a cutting wheel. This cutting wheel is used to cut a steep, thin channel or groove with steep sidewalls in the paved surfaces or solid ground. The fiber optic cable is then laid in this groove, and the cable is covered by a sealing material, such as bitumen. A foamed material can also be deposited in surrounding relationship to the cable near the bottom of the groove.
Several additional means for holding a fiber optic cable in the groove or channel are also disclosed in U.S. Pat. No. 6,371,691. One such device comprises an inverted metal channel with upturned edges that can engage the walls of the groove. This metal channel serves as upward protection and as a cable guard in addition to serving as an electrical return path for a power supply. An alternate hold down means, also disclosed in that patent, comprises an elongate solid member formed from an elastic or elastomeric material. Longitudinal ducts extend through the elastic body, and fiber optic cables can be drawn or blown though these ducts at a later time. Another duct with a slit extending along the top is also provided in the top of the profile body so that a fiber optic cable can be introduced into this top duct before the cable is laid in the channel or groove. Integral elastic barbs extend upwardly from the elastic body and engage the walls to fix the hold down device in the groove or channel. The profile body, however, does not appear to help hold the cables or the elastomeric device firmly in the channel or duct, nor is there cooperation between the cable and the hold-down device to increase retention force. In another embodiment, a strand of foam rubber is positioned above the fiber optic cable, but this strand of foam rubber is used as a release means between the fiber optic cable and filler material, such as bitumen, deposited on top of the fiber optic cable.
Although these approaches can aid in securing a fiber optic cable in an elongate groove, metal or elastomeric components can be relatively expensive. A plastic strip would be less expensive, but being plastically deformable a plastic wedge or strip having the same profiles as these metal or elastomeric devices would exert less retention force on groove sidewalls. However, a relatively inexpensive extruded plastic wedging strip would offer advantages if it could be inserted without requiring excessive force, but would still exhibit substantial resistance to movement of the fiber optic cable out of the groove. When a fiber optic cable expands longitudinally, when for example it expands thermally, the cable will bow generally in a sinusoidal fashion, and local sections can be urged upwardly out of its resident groove. External vibration and ice formation or ice heave can also cause a fiber optic cable to migrate from the groove toward the surface. These factors can be especially significant over time. Therefore, a relatively inexpensive means for holding the cable in a groove beneath the surface of a paved roadway or other solid member would be of value, especially if it can be installed without compromising current processes. The instant invention is believed to supply an inexpensive means for accomplishing this purpose.